Field of the Invention
The present invention relates to a composite substrate for a vehicle interior material, and more particularly, to a lightweight, multi-layered composite substrate manufactured by stacking a reinforcement layer formed of a natural fabric and a synthetic fabric on each of both surfaces of a thermoplastic foam sheet.
Discussion of the Related Art
A headliner is a vehicle part above the head of a passenger, which has a specific shape and is attached to a steel panel of an upper body of a vehicle. The headliner functions to protect passengers from impacts, shield them from heat, and absorb noise. Such a vehicle interior material is an important part that determines riding comfort. In general, a soft material such as resin or a fabric is used as an interior material.
Along the trend toward relatively large space and part modularization, multiple correlated parts are assembled and attached in a complex structure in the process of manufacturing a vehicle. Thus, shape stability (or dimensional stability) of materials is essential. In other words, materials should not be deformed or delaminated between layers under an ambient environment with various temperatures and humidity levels. Furthermore, due to the recent ecofriendly demands, there is a need for lightweight materials that are highly heat-resistant, strong, and highly energy-efficient.
In general, a composite substrate for a vehicle interior material is formed by stacking a reinforcement layer (or an outer surface layer) that determines a design, absorbs noise, and offers a sense of cushioning on one or both surfaces of a core layer that maintains a shape and mountability.
1) Core Layer
Conventionally, a core layer of a composite substrate for a vehicle interior material is formed of a natural fiber-reinforced board, a resin felt, a wood fiber, polyurethane foam, etc.
Specifically, as a core layer, a sheet fiber structure containing a natural fiber or a synthetic fiber is used in the form of felt for a noise-absorbing or shielding material or in the form of a natural-fiber reinforced board after it is molded into a predetermined shape through heating and molding. The conventional noise-absorbing or shielding material is not dense in structure, thus easily allowing introduction of moisture and being musty. As the noise-absorbing or shielding material is treated with preservatives to suppress decomposition of the natural fiber, generation of germs, and mold growth, toxic substances are produced. Moreover, the noise-absorbing or shielding material is not viable to a field requiring shape stability due to a low density and the resulting low strength. Although the natural fiber-reinforced board is highly strong and stable, it should be fabricated to a weight per unit area of 1200 g/m2. Therefore, the natural fiber-reinforced board is heavy and offers a low sense of cushioning due to its compressed fabrication. As a consequence, the natural fiber-reinforced board has limitations in application as a vehicle interior material.
In another example, the core layer of the conventional composite substrate for a vehicle interior material is formed of a resin felt produced by mixing hemp, cotton, and a synthetic fiber with a thermo-curable phenol resin, a wood fiber obtained by mixing wood powder with phenol resin. Although this material is strong and thus popular as an interior material for a large-sized car, its heavy weight does not help the increase of fuel efficiency, there is a smell inherent to phenol, dust harmful to the human body is generated, and a bad odor is generated from volatilization of an organic solvent during attachment to an outer surface material. As a result, a working environment gets poor and a customer may have headache while using it.
In a further example, if polyurethane foam is used for the core layer, the core layer is not laminated effectively to the reinforcement layer by heat. To laminate the reinforcement layer, an adhesive containing a primer component or a multi-layered adhesive film should be used, which makes a manufacturing process complex and increases cost.
2) Stacking of the Core Layer and the Reinforcement Layer
The composite substrate for a vehicle interior material is manufactured by stacking a mat or film formed of an inorganic fiber such as a glass fiber or a reinforced sheet formed by needle punching between a natural fiber and a synthetic fiber as a reinforcement layer on one or both surfaces of the core layer.
Particularly, reinforced sheets fabricated only by needle-punching a natural fabric and a synthetic fabric are usually stacked on both sides of the core layer in order to make a vehicle interior material lightweight and stable in shape.
However, as a reinforced sheet 202 with a weight per unit area of 300 g/m2 or below is subjected to carding and needle punching during mixing a natural fiber with a synthetic fiber, the reinforced sheet gets non-uniform in thickness and density. As a result, the properties of the composite substrate may be non-uniform and have a low flexural strength. Moreover, if the reinforced sheet 202 is formed to a weight per unit area of 300 g/m2 to make the composite substrate lightweight, the reinforced sheet 202 is very susceptible to elongation along a length direction during lamination to the foam sheet and thus its stability of a dimensional change rate is decreased after molding. If the density uniformness of the reinforced sheet 202 is decreased due to the lightweight of the reinforced sheet 202, the foam sheet is carbonized partially during pre-heating in the molding process.
In addition, the reinforced sheet formed of the natural fiber and the synthetic fiber is expanded by moisture under the condition of high temperature and high humidity and shrunk by drying. Repeated expansion and shrinkage leads to degradation of the properties of the composite substrate and deformation of the composite substrate. Particularly, if the weight of the reinforced sheet is low, it is more vulnerable to moisture. As the reinforced sheet is deformed when it is used as an interior material, there are limitations in making the interior material lightweight. Particularly, when the natural fiber of the reinforced sheet absorbs moisture under a high-temperature, high-humidity condition, the composite substrate is deformed, shrunken, or expanded, which makes it difficult to apply the composite substrate as a product.
If the core layer and the reinforcement layer are heated separately and then attached to each other by pressure during thermal molding, the thickness of the foam is reduced remarkably. As a consequence, the flexural property of the composite substrate is significantly degraded.